1. Field of the Invention
The present invention relates to a surface acoustic wave device for use, for example, as a resonator, band-pass filter, or other suitable device. In particular, the present invention relates to a surface acoustic wave device using a rotated Y plate X-propagating LiTaO3 substrate and a method for manufacturing the same.
2. Description of the Related Art
In mobile communication apparatuses, for example, cellular phones, surface acoustic wave filters have been used as RF stage band-pass filters and duplexers. As this sort of surface acoustic wave filter, the surface acoustic wave filter using a leaky surface acoustic wave has been in actual use, in which an IDT (interdigital transducer) made of Al is arranged on a 30° to 50°-rotated Y plate X-propagating LiTaO3 substrate.
However, this surface acoustic wave filter has a poor frequency temperature characteristic of −30 ppm/° C. to −40 ppm/° C. and, therefore, improvements thereto have been required. In order to improve the frequency temperature characteristic, a structure has been suggested, in which an IDT made of Al has been disposed on the 30° to 50′-rotated Y plate X-propagating LiTaO3 substrate and, thereafter, a SiO2 film has been further laminated thereon. The frequency temperature characteristic is improved by arrangement of the SiO2 film.
When the IDT made of Al is formed, the electrode film thickness H/λ (H denotes a film thickness and λ denotes a wavelength of a surface acoustic wave) of the IDT is significantly increased to 0.08 to 0.10 in order to increase the reflection coefficient and electromechanical coupling coefficient K2. Since the IDT made of Al was allowed to have a significant thickness as described above, regarding the portion shown in FIG. 16A, when the SiO2 film was formed thereon in order to improve the frequency temperature characteristic, large height differences occurred in the SiO2 film and, thereby, cracks sometimes occurred in the SiO2 film as shown in FIG. 16B. Consequently, the filter characteristics of the surface acoustic wave filter were likely to degrade due to occurrence of the cracks.
In addition, since the electrode film thickness of the IDT made of Al is large, an effect of covering the disparities (concavity and convexity) on the electrode surface of the IDT based on the arrangement of the SiO2 film was not adequate and, thereby, the temperature characteristic was not always improved adequately.
Furthermore, the attenuation constant was increased due to the arrangement of the SiO2 film and, thereby, degradation of the filter characteristics occurred.